So I spent an hour figuring out for which text portion I get this error. Apparently if any of my posts contain “/etc/” I get above 404 error. For other variations of text e.g. “/ etc/” or “/etc /” the posts get published as they should.

What could be causing this? I recently changed my host, could it be because of some module security?

It may be server-side security software, like mod_security, preventing certain things from being passed by the POST command, which WordPress uses to write to the database when publishing or editing a post.